This application relates generally to the formation of microelectromechanical devices and more specifically to protecting components of the microelectromechanical devices from galvanic degradation.
In recent years, increasing emphasis has been made on the development of techniques for producing microscopic systems that may be tailored to have specifically desired electrical and/or mechanical properties. Such systems are generically described as microelectromechanical systems (“MEMS”) and are desirable because they may be constructed with considerable versatility despite their very small size. The micromachining procedure generally uses a combination of deposition, patterning, and etching processes to produce an intermediate structure. This intermediate structure may include both “structural material” intended to form part of the final device and “sacrificial material” that is intended to be dissolved in forming the final device.
The removal of this sacrificial material is referred to as a “release,” and typically involves a chemical reaction. For example, the sacrificial material often comprises silicon dioxide SiO2, which is removed during the release with hydrofluoric acid HF. The structural material of the MEMS device typically includes materials having different galvanic potentials, such as when electrical and mechanical components are made of silicon or polysilicon are connected to metals such as gold for optical, wirebonding, and other purposes. In these cases, the HF acts as an electrolyte, and when added to the system causes it to behave as a galvanic cell. The action of this galvanic cell may cause damage to the silicon and/or gold, severely degrading the electrical and mechanical integrity of the MEMS device. In addition to such galvanic degradation, electrochemical attack of the aluminum and copper interconnects used in CMOS-integrated MEMS devices from the HF remains a persistent problem for MEMS manufacturing. Such electrochemical degradation leads to reduced product yield, performance, and reliability.
In order to avoid these harmful effects of the HF, it is common to seek micromachining methods that limit the exposure of the structural material to the HF during the release. Simply reducing the length of time the intermediate structure is exposed to the HF is often not viable because the sacrificial layers may then be incompletely removed. Accordingly, reducing the exposure of the structural material is usually achieved by adding additional fabrication steps to minimize the exposure of the structural material. The additional complexity of the fabrication process that results increases both fabrication costs and the possibility of producing defective structures. It has alternatively been suggested that the electrochemical attack of aluminum and copper interconnects in CMOS-integrated MEMS may be mitigated by using more concentrated HF, such as with a 73% HF solution instead of the more customary 48% HF solution. This method has a number of drawbacks, including the fact that such stronger-concentration HF solutions are extremely dangerous and are not generally available for commercial use.
There is thus a continuing need in the art for methods of releasing MEMS structures that limit damage caused to the structural material.